migrate multiple exchange mailbox to zimbra

Hi,
We are trying to migrate multiple exchange mailbox to zimbra 6.0.7.

With the exchange migration tool (ZCSExchangeMigrationWizard-6.0.7_GA_2504), when using the GUI, we can tune the number of mailbox to migrate concurrently (param : "Concurrency / Number of mailbox to import simultaneously")
It works well.

Now, we want to write some scripts around this tool. So we use a xml configuration file (dynamically generated) to launch the tool in "command line mode"
In the configuration.xml we specify multiple users to migrate :
<ImportUsers>
<User>
<ExchangeMail>$user1</ExchangeMail>
</User>
<User>
<ExchangeMail>$user2</ExchangeMail>
</User>
<User>
<ExchangeMail>$user3</ExchangeMail>
</User>
</ImportUsers>

But, in this mode, users are migrated only one at once, not concurrently.
Is there a way to have the same behavior with the xml configuration file than we have with the graphical wizard?

..500 users with big mailbox will take weeks to migrate if done one by one